1. Field of the Invention
The present invention relates to torque sensors, or more particularly to automotive torque sensors which measure a torque value at a point within the vehicle power train. The invention provides a method for zeroing an automotive power train torque sensor while a vehicle is moving during the essentially zero torque condition.
2. Description of the Related Art
Automotive power train control systems today use inferred torque measurement to provide critical input to a wide variety of control systems. There has long been a desire to measure engine and or transmission torque in a direct manner to improve the accuracy of the torque signal and thus provide improved engine and transmission control, for example, shift quality, gear ratio change, and speed ratio. This is typically accomplished by a mechanical control valve mechanism for modifying the pressure applied to various friction elements in the transmission and to selectively hold and release different components of a planetary gear set. The shift quality should be optimized in order to achieve a smooth transition from an original engine speed to a new engine speed. In general, it is not desirable to have a shift of very brief duration, as this produces a jerk by the rapid change in vehicle acceleration, which is very noticeable and found objectionable by most drivers. On the other hand, if the shift time is stretched out for too long a period, undue wear is imposed on the friction elements of the transmission.
It is known in the art to control power train smoothness using output torque sensing and input torque control. In this regard, see U.S. Pat. No. 4,220,058 and U.S. patent application US2005/0101435A1. An automotive power train typically has an engine, automatic transmission, multiple-ratio gearing controlled by friction elements actuated by hydraulic pressure, an output shaft torque sensor producing a signal representing the magnitude of current output torque, an electronic controller for controlling the target output torque based on the current output torque, increasing the torque capacity of the oncoming friction element and decreasing the torque capacity of the off-going friction element after a gear ratio change is initiated. During the inertia phase of the ratio change, the controller controls the engine speed to follow a predetermined rate of change of input speed. The strategy employs an electronic throttle and closed loop engine torque control and closed loop engine speed control at various phases of the gear shift, to improve shift feel. Various engine parameters, including throttle position, ignition timing, engine air-fuel ratio, and engine airflow, control engine torque and speed, are used to control input torque or input speed, depending on the shift phase.
One of the problems with on-board torque sensors is that they experience zero offset drift over the life of the vehicle. There are many components of this drift including inherent drift mechanisms in the sensor itself as well as drift mechanisms in the mechanical system that connects the source of torque, for example the engine, to the point at which the sensor is mounted in the drive train, for example the torque converter, on a transmission, on a transmission output shaft, or a differential.
It has been determined that because the various control algorithms that use the torque signal often require accurate torque measurement near the zero level, this offset drift error must be minimized. Performing a one time zeroing of the torque sensor during the production of the vehicle would not be sufficient since drift will occur as the vehicle is driven. Contributors to this drift may include temperature change, vibration, mechanical misalignment and mechanical wear of moving parts. As long as these contributors are slow changing then the methods of in-system zeroing discussed herein will be effective.
There are two main requirements that must be met in order to perform an in-system zeroing of the torque sensor. There must be a zero torque condition in the mechanical system that the torque sensor measures, and the calibration event must be transparent to the driver. This invention provides a method for using various monitored conditions of vehicle operation to determine when a near zero torque condition exists in a moving vehicle. Once this condition is determined, zeroing the sensor is done in such a way that is not noticeable to the driver. The method of zeroing is done by reading the zero torque value of the sensor output and storing it into non-volatile memory for subsequent subtraction from future torque readings. One method of creating a zero torque condition in the drive train, would be for the vehicle's transmission controller to briefly shift the transmission into neutral, read the torque sensor output, and then shift back into the proper gear. If this operation is done when the vehicle drive train is in a near zero torque condition, then the driver will not feel the calibration event. On the contrary, if this calibration event were performed when the driver is accelerating the vehicle, then the driver would experience a noticeable and unacceptable drop in power.